Downing Street said the prime minister's speech was one of a series, which include his thoughts on criminal justice, health and science.

"It shows the prime minister is interested in substance, not image, and that what he wants to do is continue to focus on the issues," his official spokesman said.

In his speech, Mr Blair said the "hard to reach" groups his "social exclusion plan" hoped to target included: children in care, families with complex problems, teenage pregnancies and mental health patients.

Mr Blair told the BBC last week social intervention could happen "pre-birth".

Clarifying these remarks, he told his audience in York "I am not talking about 'baby Asbos', trying to make the state raise children, or interfering with normal family life".

"I am saying that where it is clear, as it very often, at a young age, that children are at risk of being brought up in a dysfunctional home where there are multiple problems, say of drug abuse or offending, then instead of waiting until the child goes off the rails, we should act early enough ... to prevent it."

'Radical revision'

This was not "stigmatising the child or family". "It may be the only way to save them and the wider community from the consequences of inaction," he said.

The problem was "not that we are not trying, nor that the money is not being committed, it is that we need a radical revision of our methods".

"The social exclusion plan will be guided by five principles: early intervention, systematically identifying what works, better co-ordination of the many separate agencies, personal rights and responsibilities and intolerance of poor performance.

"More than anything else, early intervention is crucial. It is a commonplace that prevention is better than cure."

He said health visitors and midwives would seek out those most at risk by asking young parents or parents-to-be about the difficulties they may be having or about their background.

At risk

Mr Blair conceded that people hearing his proposals "will shout about the 'nanny state', who will tell us it's none of 'our business', who will say more reasonably that if you try to predict, you stigmatise.

"But today's society doesn't work like this."

He said it was not for the state to tell people they cannot choose a different lifestyle in issues, such as to do with sexuality.

"But where children are involved and are in danger of harm or where people are a risk to themselves or others, it is our duty not to stand aside," he said.

"Their fate is our business. The alternative is that these children, these adults, these families are left behind, abandoned, when they need to be helped."